1. Field of the Invention
The present invention relates to a halftone method of representing a shade of an image with a limited number of tone levels and a method of printing the shade on a printing medium.
2. Description of the Related Art
An image forming apparatus that forms an image on a printing medium, such as paper, by using a color material is used often as an apparatus that outputs an image on which processing has been performed by a personal computer, an image that has been photographed by a digital camera, etc. The number of tone levels that can be output of the image forming apparatus such as this is generally small compared to the number of tone levels of an image that is handled by a personal computer etc. Because of this, in the case where an image is output by an image forming apparatus, it is necessary to decrease the number of tone levels to the number of tone levels that the image forming apparatus can output by performing gradation conversion processing (halftone processing) on image data that is handled by a personal computer etc. As representative halftone processing, the dither method and the error diffusion method are known. Generally, the dither method is known as high-speed processing whose burden of processing is light and which can be performed in parallel. On the other hand, the error diffusion method is known as a method of obtaining an output with less feeling of granularity, more excellent frequency responsiveness, and of higher image quality compared to that obtained by the dither method, but it is difficult to perform the error diffusion method in parallel and high-speed processing of the error diffusion method is more difficult to implement compared to the dither method.
As one of the systems of an image forming apparatus that forms an image on a printing sheet by attaching a color material to the printing sheet, there is an ink jet printing system. In the ink jet printing system, it is known that an image of higher quality is obtained in the case where dots of the color material (ink) that are formed on the printing sheet are more highly dispersed. Further, in the ink jet printing system, it is known that an image of higher quality is obtained in the case where ink dots are more highly dispersed in each printing (in each path) in the system in which an image is formed by repeatedly performing printing a plurality of times on the same area (called a multipath system etc.). The reason is that, in the case where the ink is lowly dispersed, i.e., the ink is ejected concentratedly, the ink aggregates on the printing medium and an unintended dot arrangement results or ink droplets become large and there are produced areas where dots are dense and where dots are sparse on the entire printing medium. Because of this, a technique for dispersing dots in each printing (each path) in order to prevent ink dots from aggregating has been proposed (see Japanese Patent Laid-Open No. 2008-006591 and Japanese Patent Laid-Open No. 2009-262455).